Although most integrated circuits today are general purpose circuits, such as memories, processors, etc., special purpose integrated circuits are frequently needed for particular applications. Severel techniques have been developed to make such circuits. One technique makes a complete set of masks for the lithographic patterning necessary to make the circuit, i.e., it follows the techniques used for the general purpose circuits. This technique is used to make what are commonly referred to as application specific integrated circuits. These circuits are usually referred to by the acronym ASIC. Such circuits are advantageously employed in many diverse applications.
While ASICs are perfectly adequate for many applications, their manufacture requires the fabrication of a complete mask set which frequently involves substantial expense. Additionally, the manufacturing process may require substantial time. These factors are frequently not important when many integrated circuits are fabricated. However, other techniques which are either cheaper or quicker than the technique described for ASICs are desirable for circuits that are made in relatively small numbers or which must be made rapidly. One such technique makes what are referred to as field programmable circuits. Such circuits may be either logic arrays or gate arrays and are frequently referred to by the acronyms FPLA and FPGA, respectively.
These circuits are customized for their particular uses by selectively closing electrical paths in the circuit, i.e., by programming the circuit. The element used to close the electrical circuit is termed an antifuse and it is an element that changes from a high resistance OFF state to a low ON state upon application of an appropriate electric voltage. In the usual circuit, the antifuses are located directly in the memory or logic circuit. That is, the antifuse may constitute the memory element or it may be used to selectively connect various devices. This technique for programming the circuit is simple and straightforward. Programming circuits, including two high voltage transistors, are required for each bit in addition to the single antifuse. High voltage in this context means a voltage significantly higher than the 5 volts commonly used in integrated circuits.
There are, however, drawbacks to including the antifuse directly in the logic or memory circuit, as described. For example, significant chip area is required for the two high voltage transistors needed for each bit. Additionally, the antifuse ON state resistance is a critical circuit parameter and may be either too high or not easily controlled.